While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322–334, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g., less than 1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al (J. Applied Physics, 65, Pages 3610–3616, (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, also known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. No. 5,061,569, U.S. Pat. No. 5,409,783, U.S. Pat. No. 5,554,450, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,908,581, U.S. Pat. No. 5,928,802, U.S. Pat. No. 6,020,078, and U.S. Pat. No. 6,208,077, amongst others.
Notwithstanding these developments, there are continuing needs for organic EL device components, such as light-emitting materials, sometimes referred to as dopants, that will provide high luminance efficiencies combined with high color purity and long lifetimes. In particular, there is a need to be able to adjust the emission wavelength of the light-emitting material for various applications. For example, in addition to the need for blue, green, and red light-emitting materials there is a need for blue-green, yellow and orange light-emitting materials in order to formulate white-light emitting electroluminescent devices. For example, a device can emit white light by emitting a combination of colors, such as blue-green light and red light or a combination of blue light and orange light.
White EL devices can be used with color filters in full-color display devices. They can also be used with color filters in other multicolor or functional-color display devices. White EL devices for use in such display devices are easy to manufacture, and they produce reliable white light in each pixel of the displays. Although the OLEDs are referred to as white they can appear white or off-white, the CIE coordinates of the light emitted by the OLED are less important than the requirement that the spectral components passed by each of the color filters be present with sufficient intensity in that light. The devices must also have good stability in long-term operation. That is, as the devices are operated for extended periods of time, the luminance of the devices should decrease as little as possible.
A useful class of dopants is that derived from 5,6,11,12-tetraphenylnaphthacene, also referred to as rubrene. The solution spectra of these materials are typically characterized by wavelength of maximum emission, also referred to as emission λmax, in a range of 550–560 nm and are useful in organic EL devices in combination with dopants in other layers to produce white light. Use of these rubrene-derived dopants in EL devices depends on whether the material sublimes. If the material melts, its use as a dopant is limited. Sublimation and deposition are the processes by which the dopant, subjected to high temperature and low pressure passes from the solid phase to the gas phase and back to the solid phase and in the process is deposited onto the device. Depending on the chemical structure of the dopant, when the temperature needed to sublime the dopant is high, thermal decomposition can occur. If the decomposition products also sublime the device can become contaminated. Decomposition leads to the inefficient use of dopant. Contamination with decomposition products can cause the device to have shorter operational lifetimes and can contribute to color degradation and light purity. In order to achieve OLEDs that can produce high purity white light, have good stability and no contamination from dopant decomposition, in addition to efficient use of dopant, one needs to have the ability to lower the sublimation temperature.
Useful dopants are those that emit light in ethyl acetate solution in the range of 530–650 nm, have good efficiency and sublime readily.
U.S. Pat. No. 6,387,547B1; U.S. Pat. No. 6,399,223B1; and EP 1,148,109A2 teaches the use of rubrene derivatives containing either 2 phenyl groups on one end ring of the rubrene structure or 4 phenyl groups on both end rings. There is no teaching of fluorine or fluorine-containing groups on the rubrene structure.
JP 04335087A discloses specific compounds 6, 13 and 14 containing chlorine or bromine at various positions on the rubrene molecule.
WO 02100977A1 discloses compound “C12” with two heterocyclic aromatic fluorine-containing groups also on the 5- and 12-positions of the naphthacene nucleus.
JP 10289786A discloses compound “15” with fluoro- groups on the para-positions of the secondary phenyl rings at the 5- and 12-positions of the naphthacene nucleus. U.S. Ser. No. 10/700,894filed Nov. 4, 2003, describes fluorine containing rubrenes for lowering the sublimation temperature where there are certain fluoro group arrangements.
However, high sublimation temperatures and possible decomposition would limit the use of many of these rubrene derivatives. Some of these materials would also be limited in the range of hues that they could provide. Devices containing many rubrene derivatives would fail to provide consistent white OLED devices with high color purity and reduced potential for possible contamination from decomposition impurities in their deposition. It is a problem to be solved to provide an OLED device using materials that can be sublimed at a lower temperature thus providing a lowered level of decomposition during the vacuum deposition process.